The invention relates to a module for spacing at least two pipelines at a crossing point on a seabed.
When two pipelines cross on the seabed, it is preferable for the two pipes not to touch each other, for two reasons. The first is to avoid damage to the pipe coatings, which could lead to pipe corrosion. The second reason is to avoid a single point of contact between the pipes, which could lead to a concentrated load applied to the lower pipe.
One existing method for protecting a pipeline crossing consists in arranging concrete mudmats or sand bags above the first pipeline in the vicinity of the crossing point in order to create a kind of pipe crossing mattress, so that the second pipeline can be laid on top of the support and thus avoid touching the first pipeline directly.
FIG. 1 shows a conventional pipeline crossing support 10. The pipeline crossing support 10 looks like a mattress, and covers the first pipeline 12. In this embodiment it comprises concrete mudmats. The outer surface 14 of the pipeline crossing support 10 has a smooth curve to prevent excessive bending of the second pipeline 16, and to avoid the freespan effect. The freespan effect may arise when there is a space between the pipeline and the seabed 18; currents surge into this space and induce vibrations in the pipeline, which leads to fatigue problems. That is why the crossing support 10 completely fills the gap between the pipeline 16 and the seabed, up to the touchdown points which are at the opposite ends of the crossing support 10.
Generally, the length L of the crossing support 10 is on the order of several meters (between 5 and 15 m) from the first pipeline 12 to each end of the crossing support 10. This length can vary, depending on the thickness of the mattress and the rigidity of the pipeline.
The main problem of this method is that it leads to an increase of the laying cost due to the need for underwater work, which requires the mobilization of a vessel to perform various subsea tasks. As the water depth increases, the laying cost will dramatically increase with this method.
According to the invention, a pipeline crossing module is provided which can avoid these drawbacks.
According to an aspect of the invention, the pipeline crossing module may comprise:
a spacing element which may be a plastic or rubber galvanic insulation separating element or simply a tripod;
an element for loosely clamping the spacing element onto the second pipeline so that the spacing element can rotate and still remain arranged under the second pipeline due to its own weight, whereby rotation of the second pipeline during laying will not affect the downward orientation of the spacing element.
One or more of the aforesaid modules are assembled to form a pipeline crossing support for supporting the second pipeline above the first.
The pipeline crossing support may be maintained in position along the length of the second pipeline with collars clamped onto the second pipeline at each end of the crossing support. The collars may be cathodic protection collars or any other suitable type of collar. The support may comprise a plurality of modules which define a downward-facing support surface for contacting the lower pipeline. Or, the support may comprise two modules which together with the upper pipeline form a bridge over the lower pipeline.
As an alternative to being secured in place, the pipeline crossing module or modules may be allowed to slide along the upper (second) pipeline. The position of the pipeline crossing module can be monitored and adjusted for example by a Remote Operated Vehicle (ROV). This alternative may be slightly more convenient and economical, for example in cases when it is possible to install a module near the location where the first and second pipelines will cross. The module will slide along the pipeline and then the ROV will position it at the crossing point. The module will be maintained in position by the weight of the second pipeline. Thus, it may be possible to use only a single module for separating the first and second pipelines.
In deepwater laying (more than 1000 m), we can predict with an accuracy of about 12 m the position of an underwater pipe and therefore the location where a pipeline crossing region will occur.
As the predicting accuracy is around 12 m, the length of the pipeline crossing support is advantageously greater than the predicting accuracy, generally between 20 m and 40 m (60 to 120 ft). This feature of the invention assures that at the crossing point the two pipelines will be separated by a crossing module.
The thickness of the module is determined according to the required galvanic insulation, typically around 40-50 cm (18xe2x80x3).
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention, with reference to the accompanying drawings.